Haplodioritic and Related Magmas. 173 



plagioclase crystals is higher than that of the glass, in the lime- 

 rich mixtures the reverse is true. In one intermediate mixture 

 it was found that the index a of the crystals nearly matched 

 that of the glass so that by measuring the index of the glass by 

 the immersion method the composition of the plagioclase was 

 determined. It was in this manner that the three-phase- 

 boundary of 1250°, F'G-' of fig. 9, was located. At 1250° the 

 liquid in equilibrium with both plagioclase and diopside, F', 

 has a refractive index as glass at room temperature of l - 572 

 and a of the plagioclase crystals nearly matches the glass, but 

 is appreciably less (1*571), which places the composition of the 

 plagioclase at Ab,An 9 (G'). This was the only case in which 

 the composition method was applicable, for, when the indices 

 of the crystals differ considerably from the containing glass, 

 the crystals are too small for the determination of their refrac- 

 tive indices by direct comparison with immersion liquids. 



Temperature Jlethods. — Determination of three-phase- 

 boundaries by the method of finding the temperature of begin- 

 ning of melting (second method) depends on the ability to 

 detect the first trace of glass. This is a comparatively easy 

 matter in a mixture otherwise homogeneous, so that the tempera- 

 ture of beginning of melting of plagioclase mix-crystals, say, 

 can be accurately located. But in a mixture of plagioclase and 

 diopside the first trace of glass is not easily detected and there 

 would appear to be a tendency for this method to give a some- 

 what high result. Fortunately, however, this tendency is not 

 very strong since the amount of glass increases very rapidly 

 with a small rise of temperature for most compositions. (Note 

 extent of flatter portion of boundary curve, fig. 6.) 



Determination of three-phase-boundaries by this method was 

 carried out by grinding together crystalline diopside and crys- 

 talline plagioclase of definite composition and determining the 

 temperature at which the first liquid was formed. The results 

 obtained are given in Table IV. 



Table IV. 





Mixture 



Temperature of 



beginniug of 



rnelting 



AbjAnj + diopside 

 AbjAn, + diopside 

 Ab,An Q + diopside 

 AnjAn 5 + diopside 



1176° 

 1200° 

 1219° 

 1240° 



Determination of three-phase-boundaries by approaching the 

 three-phase area from above (third method) was carried out 

 with only two mixtures. In these plagioclase begins to separate 



